ELONGATED HYPOCOTYL 5 regulates light-mediated squalene biosynthesis and development in Arabidopsis thaliana

Terpenoids are diverse groups of metabolite families that are crucial for plant development and required in cosmetics and pharmacological industries. Various development processes and environmental factors, including light, have been shown to affect terpenoid biosynthesis. However, regulatory factors involved in such regulation have not been explored much. Squalene synthases (SQSs), key enzymes in the terpenoid pathway, are pivotal for sterol and triterpene biosynthesis across various organisms. In this study, we report that AtSQS1 expression and squalene content are higher in the dark, and light through ELONGATED HYPOCOTYL 5 (HY5) negatively regulates the AtSQS1 gene and squalene biosynthesis in Arabidopsis thaliana. Our study shows that the AtSQS1 gene is unaffected in the hy5-215 mutant during light and dark conditions, but it is down-regulated in WT and HY5OX lines. The histochemical GUS assays and GFP expression patterns indicate a negative regulation of squalene biosynthesis by AtHY5. Yeast one-hybrid assays, EMSA, and ChIP experiments have confirmed the physical binding of AtHY5 on light-responsive elements of the AtSQS1 promoter. We have validated our results by developing AtSQS1 promoter: reporter transgenic lines in WT, hy5-215 mutants, and HY5OX backgrounds and conducted expression, metabolite quantification, and histochemical GUS assays. The metabolites quantification of squalene and phytosterols through GC-MS further confirms that AtHY5 negatively regulates the squalene biosynthesis in a light-dependent manner in Arabidopsis.